This invention relates to a sheet clamping device in which a sheet is disposed around the peripheral surface of a drum rotating in a given direction. A variety of apparatus are known and in practical use which dispose a sheet cut to size, such as a print sheet used in a printer, an original to be transmitted or a record sheet used in a facsimile system or a sheet-shaped photosensitive member used in a copying machine, around the drum and hold it thereon.
In one type of such sheet clamping device, both the leading and the trailing end of a sheet are held against the drum surface by mechanical dumping elements. The members which hold the leading and the trailing end of the sheet are independently operable, and are urged to hold the sheet against the drum surface. When a sheet is to be disposed around and held on the drum, the latter is in continuous rotation at a relatively low speed.
When the drum around which a sheet has been disposed and held is switched to a high speed operation, a centrifugal force after causes the sheet clamping members to be moved away from the drum surface to cause a reduction in the sheet holding action thereof, thereby allowing the sheet to be disengaged from the drum surface. Such an inconvenience can be avoided by increasing the resilience with which the clamping members are urged. However, if the resilience of a spring, for example, which urges these members, is increased, there results another inconvenience in that a force of an increased magnitude is necessary to open the clamping members.
The clamping members may be opened or closed by means which may comprise a combination of a cam and a cam follower. Specifically, this means typically comprises a cam which follows the rotation of the drum and which can be selectively stopped while the drum continues to rotate, and a cam follower substantially integral with a clamping member which is pivotally mounted on the drum and engaging the cam. The cam is stopped at a given location whenever it is desired to open the clamping member. Since the drum continues to rotate at this time, the cam follower moves along the stationary cam, thereby opening the clamping member. When the leading end of a sheet is fed into the space between the open clamping member and the drum surface, the cam follower then operates to close the clamping member. At this time, the cam causes the cam follower to be driven in the same direction as the direction of rotation of the drum. However, the drum or the clamping member provided thereon may move in advance of the sheet, thus disadvantageously causing the leading end of the sheet which has once been fed into the space between the clamping member and the drum to be disengaged therefrom. Such difficulties are caused by a fluctuation in the rotational load of the drum which is in turn caused by the relative position of the cam and the cam follower as the clamping member is operated.
Of primary importance in disposing and holding a sheet around the drum is to assure that the sheet is disposed against the drum surface without any slack in the sheet. In the prior art practice, when it is desired to dispose a sheet around the peripheral surface of the rotating drum, the sheet is subjected to a combing action while its leading end is secured to the drum. This technique accomplishes the intended purpose of eliminating any slack in the sheet, but involves several difficulties when clamping the trailing end of the sheet. Specifically, if a combing member is constructed to serve as a rear clamping member simultaneously, it tends to be driven away from the drum surface under influence of a centrifugal force during the rotation of the drum, and thus a positive sheet clamping action cannot be expected. If the combing member is urged against the drum with a force of an increased magnitude, the positive sheet clamping action can be assured, but the leading end of the sheet may be forced to be disengaged from the front clamping member in the course of the combing operation. Another difficulty results when a combing member is separate from a rear clamping member. Specifically, if the combing member is arranged externally of the drum, the combing member cannot induce the trailing end of the sheet to be inserted into the space between the rear clamping member which assumes its open position and the drum surface, and cannot hold the sheet in place until the rear clamping member clamps the trailing end of the sheet. When the combing member is formed by a roller, for example, the difficulty occurs because the roller contacts the drum surface at a single point around the circumference, as viewed in the axial direction of the drum.
A conventional arrangement is provided with a sheet clamping claw which secures the leading end of a sheet. When a sheet is to be disposed around the drum, the rotational position of the drum is detected, and the sheet begins to be fed toward the drum from a sheet feeder in synchronized relationship with the movement of the sheet clamping claw. In other words, the location where the leading end of the sheet is clamped to the drum surface is fixed. For this reason, the location where the sheet clamping claw is opened in order to receive the leading end of a sheet therein, or the location where the sheet clamping operation is initiated (hereafter referred to as "first location") is fixed. A signal indicative of such position is utilized as a signal to initiate the sheet feeding operation.
Accordingly, when a button which is operated to clamp a sheet around the drum, for example, a feed initiation button, is depressed, the sheet to be disposed around the drum will immediately begin to be fed toward the sheet clamping claw if the sheet clamping claw is situated at said first location. However, if the sheet clamping claw has moved past the first location, as viewed in the direction of rotation of the drum, the drum will continue to rotate through a maximum of one revolution before the sheet clamping claw is again situated at the first location, whereupon the sheet feeding operation is initiated. Thus it will be seen that the sheet clamping operation will be initiated immediately in response to the depression of a feed initiation button or after a maximum time delay corresponding to one revolution of the drum depending on the position assumed by the sheet clamping claw at such instant.
The sheet which has been disposed around the drum is removed therefrom for delivery after the completion of a printing/copying operation. At this time, the location where the leading end of the sheet is released is also fixed. After the delivery of the sheet, another sheet is clamped to the drum. However, it will be seen that there is again a need for a waiting time mentioned above depending on the location of the sheet clamping claw.
By way of example, assume that the drum rotates at a speed of 30 r.p.m. during the sheet feeding and the delivery operation. If the rotation is started with the sheet clamping claw located past the first location, no sheet clamping operation takes place during about two seconds which is required for the drum to rotate through one revolution.
The time interval required for processing the sheet, for example, for printing or copying thereon can be reduced by increasing the drum rotation to a high value, for example, 1,500 r.p.m. or providing a plurality of printing/copying means. However, the drum rotation during the sheet feeding and the delivery operation cannot be increased to a high value, so that the proportion of the time interval required for the sheet feeding and delivery operation relative to that required for the actual printing/copying operation will increase. Thus if it is attempted to reduce the time interval for the sheet processing by increasing the speed of drum rotation during the printing/copying operation or providing a plurality of printing/copying means, the time period required for the sheet fading and delivery operation makes it difficult to reduce the overall printing/copying time.